B2.1 Membrane and Membrane Transport

Question 1

What is the membrane composed of?

Answer 1

• Lipids
• Proteins
• Carbohydrates in the form of glycolipids and glycoproteins

Question 2

3 major classes of lipids

Answer 2

1. Phospholipids
2. Glycolipids
3. Sterols

Question 3

Structure of Phospholipid molecules

Answer 3

Negatively charged phosphate molecule form the polar hydrophilic “head”
Two non-polar fatty acid chains (either saturated or unsaturated) forms the hydrophobic “tail”
Tails point inwards, head points outwards which results in a bilayer

Question 4

Definition of Amphipathic

Answer 4

Having both hydrophilic and hydrophobic elements

Question 5

Determiner for permeability of molecules

Answer 5

Size of molecules and their hydrophobic / hydrophilic nature
- As most parts of the cell are polar of charged, membranes form barriers which prevent unneeded entry or exit of molecules

Question 6

Molecules that are able to pass through the membrane

Answer 6

Permeable - Non-polar lipid soluble molecules: O2, CO2, Steroids, N2
Mostly permeable - Small uncharged polar molecules: H2O, Urea, Ethanol, Glycerol

Question 7

Molecules that are unable to pass through the membrane

Answer 7

Mostly impermeable: Large uncharged polar molecules: Glucose & Sucrose
Impermeable - Ions: Na+, K+, H+, Ca2+, Cl-

Question 8

Definition of simple diffusion

Answer 8

Movement of molecules of a substance down a concentration gradient (From a region where concentration is higher to a region where concentration is lower)
- Spontaneous process which eventually results in an equilibrium
- Passive process which does not involve expenditure of energy

Question 9

Purpose of hydrophobic nature of lipid bilayers

Answer 9

Restricts simple diffusion of most molecules, exception of non-polar molecules or very small polar molecules

Question 10

Properties of integral proteins

Answer 10

1. Embedded in the lipid bilayer
2. Amphipathic -> Causes them to be embedded in the bilayer
3. Transmembrane

Question 11

Properties of peripheral proteins

Answer 11

1. Hydrophilic in nature (Do not have hydrophobic regions)
2. Surface of membranes
3. Interact only with hydrophilic regions of phospholipids and integral proteins

Question 12

Properties & Function of Membrane Proteins

Answer 12

1. Facilitates movement in and out of cells via carrier and channel proteins
2. Recognition between cells to distinguish between self and non-self cells
3. Receptors for chemical signals
4. Enzymes to catalyze reactions
5. Cell adhesion to other cells or the environment

Question 13

Definition of Osmosis

Answer 13

Diffusion of water molecules across a selectively permeable membrane from lower to higher solute concentrations
- Always moves from region of high concentration to lower concentration until it reaches an equilibrium
- No net movement of water

Question 14

Aquaporins

Answer 14

Integral proteins that controls the flow of water in and out of the cell

Question 15

Properties of facilitated diffusion

Answer 15

1. Movement is down a concentration gradient
2. Movement is assisted by transport proteins (Channel and Carrier proteins)

Question 16

Definition of Channel Proteins

Answer 16

Transmembrane proteins that form channels / pores for passage of molecules

Question 17

Properties of Channel proteins

Answer 17

1. Highly selective
2. Different channels needed for different ions

Question 18

Selectivity in Channel proteins

Answer 18

Binding sites of hydrophilic amino acid side chains lining the channel are highly ion specific
- Most channels open and close in response to specific stimuli such as changes in voltage across membrane (Voltage-gated channels), binding of small molecules to channel proteins or mechanical forces

Question 19

Definition of Carrier Proteins

Answer 19

Undergo conformal change to transfer molecules from one side to the other

Question 20

Selectivity difference between simple diffusion and facilitated diffusion

Answer 20

Simple diffusion: Not selective, only based on size and hydrophobic/hydrophilic nature
Facilitated Diffusion: Transport proteins exhibition selectivity as they recognize specific molecules

Question 21

Active Transport

Answer 21

Transport of molecules in diffusion coupled with an exogenic reactions

Question 22

Pumps

Answer 22

Transport proteins used for active transport which can help move molecules against the concentration gradient

Question 23

Types of Active Transport

Answer 23

Direct: Energy released by exogenic reaction is used to directly transport molecules across the cell membrane
Indirect: Movement of one solid down its concentration gradient derives movement of second solute against it’s concentration gradient

Question 24

Glycolipids & Glycoproteins

Answer 24

Carbohydrates either linked to lipids or proteins

Question 25

Functions of glycolipids and glycoproteins

Answer 25

1. Cell recognition 2. Cell adhesion 3. Glycocalyx 4. Hormone Binding 5. Enzymatic 6. Channel forming 7. Pumps for active transport